Like other secretory proteins, lysosomal proteins are synthesized in the endoplasmic reticulum and transported to the Golgi apparatus. However, unlike most other secretory proteins, the lysosomal proteins are not destined for secretion into extracellular fluids but into an intracellular organelle. Within the Golgi, lysosomal proteins undergo special processing to equip them to reach their intracellular destination. Almost all lysosomal proteins undergo a variety of posttranslational modifications, including glycosylation and phosphorylation via the 6′ position of a terminal mannose group. The phosphorylated mannose residues are recognized by specific receptors on the inner surface of the Trans Golgi Network. The lysosomal proteins bind via these receptors, and are thereby separated from other secretory proteins. Subsequently, small transport vesicles containing the receptor-bound proteins are pinched off from the Trans Golgi Network and are targeted to their intracellular destination. See generally Kornfeld, Biochem. Soc. Trans. 18, 367-374 (1990).
There are over thirty lysosomal diseases, each resulting from a deficiency of a particular lysosomal protein, usually as a result of genetic mutation. See, e.g., Cotran et al., Robbins Pathologic Basis of Disease (4th ed. 1989) (incorporated by reference in its entirety for all purposes). The deficiency in the lysosomal protein usually results in harmful accumulation of a metabolite. For example, in Hurler's, Hunter's, Morquio's, and Sanfilippo's syndromes, there is an accumulation of mucopolysaccharides; in Tay-Sachs, Gaucher, Krabbe, Niemann-Pick, and Fabry syndromes, there is an accumulation of sphingolipids; and in fucosidosis and mannosidosis, there is an accumulation of fucose-containing sphingolipids and glycoprotein fragments, and of mannose-containing oligosaccharides, respectively.
Glycogen storage disease type II (GSD II; Pompe disease; acid maltase deficiency) is caused by deficiency of the lysosomal enzyme acid α-glucosidase (acid maltase). Two clinical forms are distinguished: early onset infantile and late onset, juvenile and adult. Infantile GSD II has its onset shortly after birth and presents with progressive muscular weakness and cardiac failure. This clinical variantis usually fatal within the first two years of life. Symptoms in the late onset in adult and juvenile patients occur later in life, and only skeletal muscles are involved. The patients eventually die due to respiratory insufficiency. Patients may exceptionally survive for more than six decades. There is a good correlation between the severity of the disease and the residual acid α-glucosidase activity, the activity being 10-20% of normal in late onset and less than 2% in early onset forms of the disease (see Hirschhorn, The Metabolic and Molecular Bases of Inherited Disease (Scriver et al., eds., 7th ed., McGraw-Hill, 1995), pp. 2443-2464).
Since the discovery of lysosomal enzyme deficiencies as the primary cause of lysosomal storage diseases (see, e.g., Hers, Biochem. J. 86, 11-16 (1963)), attempts have been made to treat patients having lysosomal storage diseases by (intravenous) administration of the missing enzyme, i.e., enzyme therapy. These experiments with enzyme replacement therapy for Pompe's disease were not successful. Either non-human α-glucosidase from Aspergillus niger, giving immunological reactions, or a form of the enzyme that is not efficiently taken up by cells (the low uptake form, mature enzyme from human placenta; see below) was used. Moreover, both the duration of treatment, and/or the amount of enzyme administered were insufficient (3-5). Production of lysosomal enzymes from natural sources such as human urine and bovine testis is in theory possible, but gives low yields, and the enzyme purified is not necessarily in a form that can be taken up by tissues of a recipient patient.
Notwithstanding the above uncertainties and difficulties, the invention provides methods of treating patients for Pompe's disease using human acid alpha glucosidase.